WestlawNext/Classic®: U.S. Treaties and Other International Agreements (contains Treaties and Other International Acts Series (1984-) + United States Treaties and Other International Agreements + United States Statutes at Large (vol. 8-), not PDF)
Lexis Advance & Lexis.com® database: U.S. Treaties on LEXIS (contains U.S.T. + T.I.A.S. (1985-), not PDF)
The Bluebook: A Uniform System of Citation (Columbia Law Review Ass'n et al. eds., 19th ed. 2010) has a hierarchy of preferred treaty sources:
1) United States Treaties and Other International Agreements (U.S.T.)
The Bluebook conveys the false impression that we can find U.S. treaties from 1950 to the present in the series: United States Treaties and Other International Agreements. (See Table 4.1 and Rule 21.4.5(a) of The Bluebook.) The holdings of BLS Library and many other libraries reveal, however, that the last volume of United States Treaties and Other International Agreements was vol. 35, part 6 (1983-1984). This U.S.T. volume includes treaties and other international agreements signed between 1980-1984. Additionally, the website of the U.S. Department of State's Office of the Assistant Legal Adviser for Treaty Affairs provides a date range of 1950-1982 for United States Treaties and Other International Agreements. Thus, the best U.S.-published treaty source is often a Treaties and Other International Acts Series "slip" version.
2) Treaties and Other International Acts Series (T.I.A.S.), 1945-; or prior Treaty Series (T.S.), 1795-1945, containing slip "advice-and-consent" treaties; or prior Executive Agreement Series (E.A.S.), 1928-1945, containing slip international agreements that are not "advice-and-consent" treaties.
3) United Nations Treaty Series (U.N.T.S.), 1946-
4) Senate Treaty Documents (S. Treaty Doc.), 1981- or prior Senate Executive Documents (S. Exec. Doc.)
5) U.S. Department of State Dispatch (Dep't St. Dispatch) or prior U.S. Department of State Bulletin (Dep't St. Bull.)
Regarding U.S. "advice and consent" treaties prior to 1950, The Bluebook's preferred U.S.-published source is: United States Statutes at Large (Stat.).
In the left frame of this web page, there are links to websites that provide treaties from U.S.T., T.I.A.S., and Stat.